Aluminium is generally prepared by the Bayer process by means of electrolysis of pure alumina obtained from bauxite. The aluminium oxide content of bauxite of good quality is approximately 50%. The impurities of alumina--particularly iron oxide and iron oxyhydrate being present in an amount of about 25%--are removed by treatment with alkali under pressure. The red mud formed in this step contains 40-45% of iron oxide and 10-20% of aluminium oxide. Since there is no known suitable technology for the working-up of red mud, in the neigbourhood of alumina factories several million tons of red mud are accumulated and the storage and laying of immense quantities of red mud causes serious problems.
Bauxites of lower quality having a small aluminium oxide content and a high iron oxide content can not be worked up by the Bayer process in an economical manner.
The process of the present invention enables the working up of red mud and bauxites of various quality. Thus the process of the present invention can improve the raw material supply of alumina factories and can contribute to the reduction of bauxite mining and the considerable amount of iron pentacarbonyl formed in the process can enhance the intensive development of iron powder metallurgy. Moreover, large valuable fields presently used for the storing of red mud can be used for agricultural purposes.
The problem of working-up and utilization of red mud and making bauxites iron-free has been intensively studied all over the world. Several publications and patent specifications deal with the problem of increasing the production of aluminium oxide and aluminium by reducing the iron content of bauxite. However, the said prior art methods have not been used on industrial scale so far at all or their use was very limited. This can be attributed to fact that the said known technologies are complicated, expensive, by-products are formed which can not be worked-up, large amounts of auxiliary agents are required, the energy-need is high and the processes are not selective.
According to a wide-spread view, red mud is a potential secondary raw material source [Thakur, R. S., Sant, B. R.: Chem. Era, 1980 16 (5) 106-7; Zimmer, E.: Aluminium (Dusseldorf) 1980, 56 (10), 639-42].
According to other process [Yoshii Chikao, Ishimra Koutaro, Hokkaido Daigaku Kogakubu Kenkyu Hokoku, 1978, (89) 1-6] red mud is roasted in the presence of calcium oxide as slag forming agent at a temperature of 1450.degree. C. thereafter the roasted product is treated with a melted alkali and red mud and aluminium are dissolved in the form of NaAl.sub.2 O.sub.2.
According to a further process (Matyash, V. G., Kudinov, B. Z., Leontev, L. I.: Tr. Inst. Metall., Akad. Nauk USSR. Ural. Neuchn. Tsentr. 1977, 30, 103-5) the starting material is roasted with calcium oxide at a temperature of 1100.degree. C. and this step is combined with reduction by semi-coke. Thus 80% of the iron content can be removed.
According to still further process [Ejima, Tatsuhiko, Shimakage Kazuyoshi, Hoshi Nasayoshi: Keikinzoku, 1978, 28 (9) 443-9] roasting is accomplished at 450.degree. C. with NH.sub.4 HSO.sub.4. Aluminium and iron are dissolved from the roasted product with sulfuric acid.
According to a further process [Logomerac, V. G.: Trav. Com. Int. Etude Bauxites, Alumine Alum. 1979, 15, 279-851] roasting is carried out in an electric furnace whereupon the metals are dissolved with 30% sulfuric acid and the valuable components are recovered by extracting with bis-2-ethyl-hexyl phosphoric acid.
According to an other method roasting is carried out at 400.degree.-1000.degree. C. in the presence of FeSO.sub.4 and the sulphates formed are separated from SiO.sub.2 by dissolving them in water [Mitsui Alumina Seizo K. K.: Jpn. Kokai Tokkyo Koho 8177,309 Nov. 29, 1979].
Several procedures are based on using strong acids, e.g. hydrochloric acid, sulfuric acid or sulfur trioxide [Zimmer, E.: Aluminium (Dusseldorf) 1980, 56 (10) 639-42; Hungarian Pat. No. 150,459; U.S. Pat. No. 3,185,545; Hungarian Pat. No. 179,799]. According to these processes dried and ground red mud is treated with the acid in a counter-current and the metal salt formed is converted into the corresponding oxide by roasting.
A very interesting method is disclosed in British Pat. No. 2,078,211. Neutralized red mud is separated into two fractions by the aid of a magnet; one of the fractions is enriched in iron, while the other has a low iron content.
In the Bayer-process the iron content of bauxites having a high iron content can be reduced on the account of increasing the amount of lime by 15-20% [Pauker, V. I., Zubarev, V. I., Simakova, L. G. (USSR) (Tsvetn) Tsvetn. Met. 1980, (7) 79-83].
According to an other group of procedures iron is removed as iron chloride. Bauxite is dried and calcinated at an elevated temperature of 600.degree.-700.degree. C., ground and treated in counter-current with hydrochloric acid or gaseous chlorine. In the said process aluminium is also converted into chloride and is to be separated from the iron, titanium, magnesium, calcium and silicium chlorides by fractionated distillation or can be selectively dissolved from the above compounds [Zotikova, A. N., Kozlov, V. M., Vinkelberg, V. G., Guseva, N. S., Pavlova, L. M.: (USSR) Ref. Zh. Metall. 1979. Abstr. No. 12 G 175.; Foley, E., Wadsley, M. W.: British Pat. Spec. 2,023,113 Dec. 28, 1979; Zotikova, A. N., Vinkelberg, V. G., Pavlova, L. M., Minina, K. P.: (USSR) Ref. Zh. Khim. 1982. Abstr. No. 1 L 98.; Kapoly L., Szabo Lne, Czegledi B., Stocker L., Riederauer Sz., Stocker L.: "Hazai bauxitok vastalanitasa" Tatabanya, Nov. 1, 1982].
According to certain procedures the aluminate solution formed in the Bayer-process is treated with hydrochloric acid and iron is separated from this solution by the aid of organic phosphates or oxidizing agents [Ni, L. P., Savchenko, A. I.: (USSR) Kompleksn. Ispolz. Miner. Syrya, 1980, (6) 81-83.; Cocco, A., Colussi, I., Kikic, I., Meriani, S.: Int. Solvent Extr. Conf. (Proc) 1980, 3. Paper 80-186, 7 pp.].
According to works published by Hungarian researchers bauxite having a high iron content or bauxite enriched with red mud is treated with ammonium chloride at a high temperature and thereby iron is removed in the form of Fe(OH).sub.3 or FeCl.sub.3 [ZamboJ., Molnar L., Siklosi P.: Banyasz. Kohasz Lapok Kohasz, 1980, 113 (6) 270-3.; Zambo J., Molnar L., Siklosi P.: Trav. Com. Int. Etude Bauxites, Alumine Alum. 1981, 16, 183-92].
According to the said procedures about 70-80% of the iron content can be removed in a very complicated manner and in most cases aluminium is obtained as chloride.
Several publications deal with the preparation of iron pentacarbonyl. However, none of the references relate to the preparation of iron pentacarbonyl from red mud or bauxite.
Iron pentacarbonyl was discovered in France in 1891 by M. Berthelott [Berthelott, M.: Compt. rend. 112, 1343 (1891), 113, 679 (1892)] and independently from the above author in England by Mond et al [Mond, L., Langer, C.: J. Chem, Soc. 59, 1090 (1891)]. For obtaining iron pentacarbonyl iron powder obtained by the reduction of iron oxalate was reacted with carbon monoxide under atmospheric pressure. In order to improve the low yields the CO-pressure was increased up to 300 bars; but however they failed to reach a conversion of 100%.
According to Mittasch A. [Mittasch, A.: Z. Angew. Chem. 41, 827 (1928)] and Hieber et al [Hieber, W.: Metallcarbonyle, FIAT Review. Inorg. Chem. Part. II. 108-145 (1946)] very good results were obtained by using Raney-iron as starting material.
Reppe et al prepared iron carbonyl from iron sulfate at high temperature and under high pressure [Reppe, W.: Ann. Chem. 1953. No. 582, 116-121].
Industrial scale production of iron pentacarbonyl is carried out by starting from iron powder--obtained by reduction of iron oxides--at 180.degree.-200.degree. C. under a CO-pressure of 200 bars [Szirkin, V. G.: Karbonylnie Metalli, Moszkva, 1978 p. 98].